I have previously published a description of Hodgepodge, the mish-mash mount I used for the solar eclipse. While I wrote about the mount, and posted some photos, I did not really cover the construction. Now for some details…
With the solar eclipse looming on the calendar I realized I needed a tracking mount to allow photography. Tracking would allow me to keep the Sun and Moon in the field without wasting precious seconds framing the image during the eclipse. It would also enable longer exposures without motion blurring the image.
It is tempting to call it Frankenscope after the similarities with the classic monster.
To further add the the Mary Shelley plot similarities, much of this was revived from the junk pile. I very nearly decided to toss the drive and fork, they were that bad, peeling paint and rusty bolts. A great deal of work was necessary to re-animate these components.
The wedge came from a telescope that was killed in an unfortunate incident with an aquarium heater. The heater was used to de-humidify the OTA and prevent fungus on the optics. Note: Aquarium heaters are not made to operate out of water.
Several new aluminum parts were machined from scraps, some of which were scavenged when the observatory shop was being cleaned out and a lot of metal stock was tossed.
Many of the electronic components used to build the drive corrector were also scavenged parts from dead electronics, this includes the 1.8432MHz crystal that forms it’s beating heart. This heartbeat keeps the mount turning at exactly sidereal rate.
Telescope clock drives from the 1980’s or earlier often used AC synchronous motors. These commonly available AC motors are used to power timeclocks, record player turntables, and telescopes, anyplace a motor needed to run at a very accurate speed.
The speed of a synchronous motor is set by the frequency of the powerline, in North America and many other places this is 60Hz. As the frequency must be synchronized for every power station on the grid the frequency is quite accurate, a feature exploited by clockmakers and telescope builders. Once found everywhere these motors are less common, but are still around.
It was the common use of these motors in telescope drives that led to the invention of the drive corrector, a device that was once a required piece of kit for serious amateur astronomers. Drive correctors like this were needed when operating from a battery at some remote location, generating AC from a 12Vdc car battery.
You also needed a drive corrector for guiding while doing astrophotography. The corrector could speed up or slow down the telescope drive a bit to correct the telescope drive speed and stay on target, something not possible with the fixed 60Hz of the mains supply. Thus the term drive corrector.
In restoring a 20″ Obsession telescope I found myself pulling a book from the shelf that had not been opened in a while. David Kriege and Richard Berry’s The Dobsonian Telescope – A Practical manual for Building Large Aperture Telescopes is a book I once read cover to cover.
The information here was critical in the success of my building Deep Violet, my 18″ telescope. Within the pages of this book are plans and drawings of the important bits as well as detailed discussions of what does, and does not work, when building a telescope.
The Dobsonian Telescope is the primary reference for those building large amateur telescopes. This book, along with the design revolution that went with it, put large telescopes in the hands of countless amateur astronomers. These telescopes extended the capabilities of amateur observers immensely, allowing spectacular views of deep space objects that were only fuzzy smears in the eyepiece before. Want to see the spiral arms of galaxies? A 20″ telescope can do that!
As I perused my well thumbed copy I was surprised to find bits of my own telescope plans used as bookmarks. There was dust on the top of the pages, but I still remembered where I disagreed with Kriege in the dimensions of the mirror box, or how to place the truss tube clamps. I may have deviated from the plans shown here in some aspects, but in other parts of the design I directly used the dimensions shown in the book.
So many old memories, good memories. Pursuing an art that has been around for four centuries, combining bits of wood and glass to make an instrument that can reveal our universe. Sure you can simply buy a very good modern telescope. But it is hard to overstate the pleasure of building one yourself. This is an art that can still be done in a garage, with tools available at the local hardware store, with results that can rival or even surpass something purchased from a catalog.
I have already posted about the restoration of an orange tube C8. That was only part of the story, the telescope is paired with a TeleVue Renaissance mount that was in the same poor condition as the optical tube. The mount required the same treatment, a complete tear down and restoration to reverse the ravages that tropical humidity had wrought upon the metal parts.
Corrosion was the issue. While the mount was mostly functional, it was looking horrible. The aluminum under the paint was corroding into a fine white powder. Most of the screws were quite rusted along with the counterweight shaft where the chrome was flaking away and the rust spreading. If allowed to continue the mount would soon be the piece of junk it looked like. There was something wrong with the clutches as well, they do not lock firmly and needed to be inspected.
The mount was sold by TeleVue in the 1980’s paired with their APO refractors. The mount is actually a re-labled unit manufactured by the Japanese firm Carton Optics as the model Super Nova or RSM2000. Well regarded by amateur astronomers you can find postings of well used and beloved mounts still in use thirty years later. Examining the mount I find I agree with those who like it. There is much to love in the solid smooth motion, this looks to be worth the effort of restoration.
Thus I set about the task of stripping down the mount into component pieces… Of the rusted hardware only one screw required drilling out, the hex drive head stripping when I tried to remove it. Fortunately I again had the proper tools, a set of easy outs to remove the remains of the screw after I had drilled the head off. I was able to remove the screw without any damage to the aluminum castings.
It looked horrible! The paint was coming off everywhere with heavily corroded aluminum underneath. Most of the screw heads were small balls of rust, with hopefully enough remaining to fit a screwdriver to and remove. For a precision optical instrument this small telescope was not very encouraging.
On the bright side the optics looked to be in decent shape. While there was some dirt and mold on the corrector, the primary looked almost perfect. Not bad considering the condition of the metal parts. Maybe, just maybe, this telescope is salvageable.
The telescope in question is an old orange tube Celestron C8. Thousands of these little telescopes were manufactured in the 70’s and 80’s. It was the C8 that set the standard for amateur telescopes at the time. The C8 is still in production forty years later, but the tubes are no longer painted orange as they were originally. Compact, yet offering decent performance, these telescopes were well regarded and hold a special place in the memories of many amateur astronomers. I have seen these little orange telescopes at dozens of star parties, even bolted to the side of huge professional telescopes for use as finders.
The SBIG ST-i is a useful little camera, I have enjoyed using mine. I usually use the camera for autoguiding, but it can also be used for basic astrophotography and even some science. In preparation for using the ST-i with a group of students I had need to make a couple additional mounts. After I go to the effort of designing a simple solution to my problem I may as well share the solution.
The ST-i camera is designed to slip into a standard telescope focuser in place of an eyepiece, as it is the same diameter at 1.25″. The camera can also be used in a “piggyback” style, mounted atop a telescope and fitted with a standard c-mount lens. Using a lens the camera will have a much larger field of view. The front of the camera is threaded for c-type 1″ threads to accomodate this. The SBIG guiding kit provides such a lens and a ring style mount. If you do not have the kit, or already have a suitable c-mount lens you still need a suitable mount.
I have included the mechanical drawing for the mount in the link above. I machined this from a block of aluminum. There is no reason it could not be made of wood or plastic to allow fabrication with whatever equipment is available. For wood you may need to make the block a little longer and use inserts for the threading. Plastic could be done pretty much as drawn.
The version I made was milled from a solid chunk of aluminum, but a good version could be easily cut from wood and assembled with brass inserts. The design could also be 3D printed without much loss in mechanical robustness.
To mount a c-mount lens you will need the adapter ring sold by SBIG to convert the 1.25″ filter thread found on the camera to the 1″ c-mount thread. Still, at $40 this ring is a lot less than the $350 guiding kit. Good c-mount lenses can be found from many sources for less than $100. You will need a focal length between 75 and 150mm for a nice image scale and as wide an f/ratio as you can find. The kit includes a 100mm f/2.8 lens which I find is quite useful in guiding my Televue 76mm or the AT6RC.